The term "signature analysis" has been employed in connection with test and quality control systems where respective specific characteristics, i.e., a signature, of a test piece are measured and compared to predetermined expected (reference) values. For example, a "signature analysis" system for testing printed circuits is described in U.S. Pat. No. 4,527,272 issued on July 2, 1985 to M. G. Reiney. Signal measurements are taken at a plurality of test points on the circuit board test piece and each compared to expected signal values.
In a stamping press, respective upper and lower portions of a die are forced together to effect various stamping operations, such as forming, cutting, coining, bending, punching or drawing, on a workpiece, typically sheet metal, disposed between the die portions.
Systems for monitoring the force exerted during the stamping process are, in general, known. For example, various commercially available systems employ sensors fitted into respective support members of the press frame for visual display of the force curves as a setup aid (to equalize force on the respective support members). Some such systems also generate an alarm or disable the press in the event that the total forming tonnage (maximum force exerted by the press) exceeds a predetermined level. An example of such system is the Load Guard, marketed by Helm Instruments. However, dies typically include a relatively large number of individual stations for performing the various forming operations and such systems do not provide information regarding the specific portion of the die at which the problem occurs.
It has been proposed to dispose sensors on a stamping die to monitor individual "critical stations" on the die and disable the press in the event of an unacceptable force variation associated with a specific forming operation. Total press load (on respective supports) is also monitored and displayed as a function of time to aid in setup (balancing for die) and for selecting optimal points of control, i.e., identifying "critical stations." (See "In-Process Control Can also be Used on Stamping Presses," Brankamp Journal in November, 1987.) However, since most dies include a relatively large number of forming operations, such systems tend to require a prohibitively large multiplicity of sensors. In addition, the system must be reconfigured, and disposition of sensors re-effected, each time the dies are changed.